1. Field of the Invention
The present invention relates to a ballast for a discharge lamp, which can make a sustained power supply to the discharge lamp, and method and apparatus for manufacturing the same.
2. Description of the Prior Art
The ballasts may be classified at large into open type ballasts each of which core assembly is exposed to air and closed type ballasts each of which core assembly is sealed within a case by a stuffing material.
As their core assemblies are exposed to air, the open type ballasts are widely used in discharge lamps for domestic use or office use in which foreign materials, such as dusts do not affect, while the closed type ballasts are widely used in discharge lamps for outdoor signboards, moist or dusty shops, or places in which safety is required.
The ballast includes a core assembly having a bobbin wound with copper wire, into which bobbin two stacks of a plurality of cores each having an "E" form are inserted in opposite direction so that both ends of the opposite "Es" are come into contact while the central ends of the opposite "Es" leave a gap between them.
The most important performance of the ballast, that is a stable supply of power to a discharge lamp, is decided by the gap between the central ends of the cores of the core assembly.
FIG. 1 illustrates a perspective view of a conventional open type ballast, of which structure and manufacturing method will be explained, hereinafter.
A core assembly 4 includes a bobbin 2 wound with copper wire 1, into which bobbin 2 stacks of a plurality of cores 3 are inserted oppositely.
The core assembly 4 is, held by downward bent parts, fixed within a retainer 5.
To manufacture this ballast, the copper wire 1 is wound around the bobbin 2 for predetermined number of times as a preparation, and a plurality of the "E" formed cores 3 are stacked. Then, one pair of the stacked cores 3 are inserted into the prepared bobbin oppositely to form a gap G between the central ends 3a at a central part of the contact parts(R1).
This bobbin 2 with wound of the copper wire 1 and insertion of the cores 3 is called a core assembly.
When this core assembly is placed in a retainer 5 and both of upper edges of the retainer are pressed down by a press, both of the upper edges are bent inside to form the downward bent parts 5a, which hold upper surfaces of the cores 3 to fix the core assembly in the retainer 5(R2).
Then, the finished open type ballast is dipped and dried(R3).
FIG. 2 illustrates a perspective view of a disassembled conventional closed type ballast, and FIG. 3 illustrates a longitudinal sectional view of a assembled conventional closed type ballast shown in FIG. 2, in which it is shown that the open type ballast of the aforementioned structure is placed in a case 6 of which one side is opened and inside of which is stuffed with a stuffing material 7 and the opened part is closed by a cover 8.
A method for manufacturing the closed type ballast of the aforementioned structure will be explained.
First, the copper wire 1 is wound on an outer circumference of the bobbin 2 for predetermined number of times as a preparation, and a plurality of the "E" formed cores 3 are stacked. Then, one pair of the stacked cores 3 are inserted into the prepared bobbin 2 with both of the central ends 3a of the stacks to face inside of the bobbin 2 to form a core assembly 4(R1).
When this core assembly 4 is placed in a retainer 5 having upward bent pieces and upper edges of the upward bent pieces of the retainer 5 are pressed down by a press, both of the upper edges are bent inside to form the downward bent parts 5a, which hold upper surfaces of the cores 3 to fix the core assembly 4 in the retainer 5(R2).
After the core assembly 4 is fixed to the retainer 5 through aforementioned process, this is dipped(R3), placed into a case 6(R4) of which upper side is opened, sealed with the stuffing material 7(R5), and closed with a plate cover 8, thus, to finish the manufacturing of the ballast(R6).
However, the conventional open and closed type ballasts have the following problems.
First, the downward pressing on the upward bent pieces in the retainer 5 to press down upper surfaces of the cores 3, especially when the pressing down force is not constant, may cause the dimension of the gap G, which is the most important factor for a ballast performance, incorrect, resulting in assurance of the quality reproducibility being impossible.
That is, if the pressing force is weak, the contact between both ends can not be assured, and, contrary to this, if the pressing force is great, since the cores 3 that come into contact can not be maintained vertical to the other, but inclined to either one side, that reduces the gap G, the electrical performances of the ballast(characteristic values, inductance, and etc.) can be degraded.
Second, the exposure of the upper side of the cores 3 to air causes much noise during its operation.
Third, in order to seal the core assembly with the stuffing material 7, since the core assembly should be fixed by means of the downward bent parts 5a on the retainer 5, placed into an additional case 6, sealed by the stuffing material 7 and enclosed by the cover 8, the required many number of components and manufacturing process causes, not only the productivity low, but also the cost high.
Fourth, since the electrical performance test of the open type ballast should be carried out before the sealing by the stuffing material of the open type ballast placed in the case 6, to select good quality ones, the working efficiency has been low.